A touch panel (TP) as an input media, can be integrated with a display screen to form a touch screen, which plays an important role in the display field.
Wherein, a capacitive touch screen, due to its high sensitivity, has attracted much attention. A mutual capacitive screen, due to its high sensitivity and an advantage of multi-point touch control, has gained much popularity. The capacitive touch screen includes out of cell touch screen and embedded-type touch screen. The embedded-type touch screen is one that integrates a TP into a liquid crystal display panel. Because of the low cost of manufacture, good light transmission and relatively thin module thickness, the embedded-type touch screen is becoming a focus of study.
The basic operation principle of the mutual capacitive touch screen will be briefly described. A coordinate in X-direction of a touch point is determined by a touch driving electrode of a mutual capacitive touch screen, and a coordinate in Y-direction of the touch point is determined by a touch sensing electrode of the touch screen. A touch driving voltage is applied on the side of the touch driving electrodes while a constant voltage is applied on the side of the touch sensing electrodes. To detect the touch points, the touch driving electrodes in X-direction are scanned progressively; when each row of the touch driving electrode is being scanned, signals on all columns of touch sensing electrodes are read; and all intersections of rows and columns can be scanned by one circle of scanning. This way of touch control location detection may particularly determine coordinates of multiple points and thus achieve multi-point touch.
In the development of nowadays embedded-type touch screen technology, a growing trend is to multiplex the existing lines of a liquid-crystal display panel as touch sensors, and implement display and touch functions by a time division driving. That is to say, during a display stage, each signal charges and discharges according to an original liquid-crystal display panel driving manner; and during a touch stage, all pixel TFTs are turned off, a voltage across two ends of the liquid-crystal remains constant, and the multiplexed lines perform touching-related operations.
A current touch screen uses a common electrode (Vcom) as the touch driving electrode, Vcom performs a high-frequency-pulse-scan as a touch scan line while pixels remain in a liquid-crystal state. In this way, because of coupling, a pixel voltage varies following variation of the voltage of Vcom, and the magnitudes of the variations of the pixel and Vcom are different due to the presence of a pixel capacitance. When this is reflected to an effective (RMS) voltage, it shows that a difference between the effective values of the pixel voltage and Vcom is smaller than that in the non-scanning state of Vcom.
As shown in FIGS. 1 and 2, timing diagrams of a touch screen driving method in the prior art are demonstrated. In the prior art, the touch operation is performed after all data are displayed. The touch operation may cause a disturbance of voltage, and the magnitudes of the variations of the pixel electrode and Vcom are inconsistent due to the presence of the pixel capacitance, such that quality of the picture is affected. Each data frame may be divided into a display period and a touch period. At the display period, pixel electrodes are charged, the common electrodes remain at a DC/AC voltage, and a voltage difference between the pixel electrode and the common electrode remains constant due to the presence of the storage capacitance. At the touch period, the common electrode, as the touch driving electrode, performs a high-frequency scan with different frequency and voltage from those during the display period, and because of the presence of the storage capacitance, the voltage difference between the pixel electrode and the common electrode during the touch period can not be consistent with the previous voltage difference, which will result in flickers in the picture. In addition, as the touch and scan are put together, the driving of the touch screen is susceptible to the disturbance of the external noise.